Protein trafficking and transport is a crucial aspect of cellular function because cells interact with the environment by internalization and release of proteins from the cell surface. Aberrant processing or transport of cell surface proteins contributes to diseases such as diabetes insipidus, cystic fibrosis, and Alzheimer's disease. While the particular cargo such as channels and receptors is unique to different cell types, the process of plasma membrane recycling is highly conserved among mammalian cells and regulated by a common set of trafficking proteins including the Rab small GTPases. Rab11 is a ubiquitously expressed small GTPase involved in recycling that interacts with the Rab11-Family Interacting Proteins (FIPs). Although the functions of FIPs are not fully understood, they may in part define distinct subdomains within the recycling system. Furthermore, preliminary data suggest that FIP2 exists in different conformations in different subcellular localizations. These data lead to the hypothesis that discrete oligomers of FIPs interacting with Rab11 allow for temporal and spatial specificity of Rab11 and FIP function within the recycling system. Two aims are proposed to test this idea. First, homodimerization and heterodimerization capabilities of FIPs will be assessed by yeast two hybrid, FRET microscopy, and protein colocalization as a way to understand FIP function within the recycling system. Second, FIP2 conformation will be studied with regards to intramolecular and intermolecular interactions within the recycling system. By elucidating the composition of FIP complexes and their subcellular localizations, the importance of Rab11 and FIPs as general regulators of plasma membrane recycling may be uncovered. SIGNIFICANCE: Proper protein transport and localization are crucial to maintenance of normal cellular function. Diseases such as cystic fibrosis, diabetes insipidus, and Alzheimer's disease result from mislocalization or aberrant trafficking of vital proteins. This proposal aims to increase the understanding of the basic mechanism of protein trafficking that is common to all cell types by studying an important trafficking protein, Rab11, and its interacting proteins.